Hydrogen bonded С–H···Y (Y = O, S, Hal) molecular complexes: A natural bond orbital analysis

Resumo

Hydrogen bonded C–H···Y complexes formed by H₂O, H₂S molecules, hydrogen halides, and halogen-ions with methane, halogen substituted methane as well as with the C₂H₂ and NCH molecules were studied at the MP2/aug-cc-pVDZ level. The structure of NBOs corresponding to lone pair of acceptor Y, n_Y, and vacant anti-σ-bond C–H of proton donor was analyzed and estimates of second order perturbation energy Е⁽²⁾ characterizing donor–acceptor n_Y → σ_C-H^* charge-transfer interaction were obtained. Computational results for complexes of methane and its halogen substituted derivatives show that for each set of analogous structures, the Е_nY→σ*C-H⁽²⁾ energy tends to grow with an increase in the s-component percentage in the lone pair NBO of acceptor Y. Calculations for different C···Y distances show that the equilibrium geometries of complexes lie in the region where the E⁽²⁾ energy is highest and it changes symbatically with the length of the covalent С–H bond when the R(C···Y) distance is varied. The performed analysis allows us to divide the hydrogen bonded complexes into two groups, depending on the pattern of overlapping for NBOs of the hydrogen bridge.